A typical protection function is based on a test value which is either a measured value such as a current or voltage value or a value derived from the analysis of a number of measured values. The test value could also be a non-electrical quantity such as a temperature or vibration amplitude. To determine if an action or no action is to be taken, the test value is compared to a threshold value. If the test value is on the dangerous side of the threshold, an action is taken. For example, a trip signal is initialized to a circuit breaker, which in turn is operated in a way to prevent the other devices in the electrical power system from damage. In simple cases, the threshold is just a fixed value; while in more intricate protection functions, it may be dependent on time and on other measured values. The principle is however the same. The quality or accuracy of the measured value may change due to various factors; for example, in the electrical power system, different voltage levels, current ratings and grounding methods may provide measured values with different accuracy levels.
An improper action based on the measured values may generate an unexpected result. For example, in a case where an action that should be taken in response to a fault is not initialized, damage will be caused, as a consequence, to the devices in the electrical power system. Therefore, protection functions are usually designed for high dependability. A common problem with a protection function possessing high dependability is that the protection function may be too sensitive and insecure, which gives rise to a tendency to take an unwanted action, which may, for example, unnecessarily block electricity production in a nuclear power system to result in an unexpected cost for the user of the system.
Today, by testing the outputs of the protection function, the designer of the protection function, commissioning engineers and customers may set a threshold value. This, however, does not ensure that the threshold is properly set, and, as consequence, an improper threshold value may result in a number of false actions. Furthermore, this approach is time- and cost-consuming due to the variation of test values that are required. Moreover, users have no indication as to under which circumstance a false action will appear until a number of false actions have been observed and the operation of an electrical power system has been disturbed.
Therefore, it is important to be able to reduce the number of false actions in an optimal way to avoid unmotivated disturbances in an electrical power system.